Many people have hang ups over negatives, but there's no need to. If I ask you to walk three steps forward, you can do it, no problem. That's positive, walking forward. If I ask you to walk four steps back, no problem. That's negative, walking back. And you would end up one step back from where you started, that's negative one or -1. In the question, you walk 21 steps forward and 21 steps back, so you're back to where you started, no steps in either direction (zero), then you take four steps forward and that's where you end up, just 4 steps forward, or positive 4. So the answer is 4.